SUNDAY, June 20 (HealthDay News) -- Children exposed early in life to polychlorinated biphenyls (PCBs) may have a reduced immune response to tetanus and diphtheria vaccines, a new study indicates.

The findings suggest that exposure to the environmental pollutants in the first years of life (a critical time in immune system development) could undermine the effectiveness of childhood vaccinations and possibly weaken immune system responses to infection, the Danish and U.S. researchers reported.

They studied 587 children born between 1999 and 2001 on the Faroe Islands, located in the North Atlantic between Norway and Iceland. Residents of the islands have widely varying PCB exposure because of different patterns of consumption of PCB-contaminated foods, such as pilot whale blubber.

The children's mothers provided blood samples at 32 weeks of pregnancy and breast-milk samples four to five days after birth. The children were vaccinated against tetanus and diphtheria at 3, 5 and 12 months and received booster shots at 5 years. Blood samples were collected from the children at 12 or 18 months, before and after vaccinations at age 5, and again at age 7.

The blood and milk samples taken from the mothers were analyzed for PCBs, and the children's blood samples were analyzed for PCBs and/or diphtheria and tetanus antibody concentrations.

The researchers found that higher PCB concentrations, particularly in children at 18 months, were associated with lower concentrations of diphtheria and tetanus antibodies at ages 5 and 7. Some children had antibody levels lower than what's needed to protect against the two diseases.

The study was published online June 20 in advance of print publication in the journal Environmental Health Perspectives.